The Influence of Kelvin Waves during Dry and Wet African Rainfall Years

Synoptic scale Kelvin waves (KWs) and easterly waves are known to have an important influence on deep convection and atmospheric low-level circulations in the tropics. Over tropical Africa, KW activity contributes to about 5-10% to the total convective variability during the boreal summer and about 15% during spring. In comparison, African easterly waves (AEWs) contribute to about 35% to the total convection during summer over land. The KWs originate or enhance in association with organized deep convection over equatorial East Pacific and propagate eastward and impact convection over Africa. Well-organized deep convection over equatorial East Pacific is associated with warmer than normal sea surface temperatures (SSTs). However, warmer than normal SSTs over equatorial eastern Pacific are related with drier than normal rainfall years over North Africa during the boreal summer. On the other hand, AEWs originate over Central and East Africa and they are key African weather systems during summer. AEWs are active and more frequent during the wet summer years as their initiation is associated with deep convection over eastern Africa. Dry season AEWs are less frequent and weakly coupled to deep convection. Therefore, we hypothesize that the dry season summer rainfall over Africa is more influenced by KWs rather than AEWs. Preliminary evidences show that KW are more active during dry African rainfall years than wet years and, thus, a significant part of the summer rainfall in dry years is related to active KWs. In contrast, KWs are less active and its impact on convection is weak during wet years.